The way such a cut-off is defined varies depending on the regulations in force in various different countries; however, for dipped beams there are two main types of standard.
A first very widespread standard is the European standard to which the light beam is delimited by a horizontal half-plane situated to the left of the horizontal axis of the headlight (for righthand drive), and by a half-plane situated to the right of the same axis, and inclined at a small upwards angle of 15.degree. from said axis.
For further details concerning this standard, reference should be made to the Official Journal of the European Community, number L 262, dated Sept. 27th, 1976, page 108.
A commonly used headlight capable of emitting a main beam and a dipped beam of the type specified above comprises a closure glass having elements for deflecting light by refraction, a parabolic reflector, and a lamp comprising two axially disposed filaments, with the rear filament being used for the main beam, and with the front filament which is provided with a masking cup being used for the dipped beam.
The focus of the reflector is situated between the two filaments so that light rays from the dipped beam as reflected from the rear portion of the reflector are initially converging rays.
Unfortunately, this convergence produces a very high concentration of light at the center of the glass together with considerable heating. In practice, if the glass is made of transparent plastic material it will inevitably become deformed.
This phenomenon is accentuated when the closure glass is at an increased distance from the lamp and the reflector, for design reasons.
Alternatively, in order to satisfy the other very widespread standard which is applicable in the United States of America and which is referred to as SAEJ 579C, it may be required that the dipped beam should lie below a cut-off limit defined by two horizontal half-planes which are at slightly different heights on either side of the forward axis.
Further, in order to obtain high light yield, the Assignees have proposed, in their French patent application published under the No. 2 583 139, a headlight which provides a dipped beam of this type and which has a reflector which is complex in shape and which co-operates with an axial filament and which is suitable for forming images of the filament below the cut-off, thereby making it possible to use smaller focal lengths and consequently to recover a very much greater quantity of light flux.
Finally, French patent application No. 86/07 461 filed May 26, 1986 in the name of the present Assignees, describes a dipped beam headlight whose beam also has a cut-off of this type, with the concentration spot of said beam being offset to the right from the headlight axis by a novel definition of the reflecting surface, i.e. without requiring any corresponding tilting of the reflector and of the lamp.
However, in all of these headlights satisfying the American standard, there still remains the problem of the center of the glass becoming heated, and this is due to the fact that the rays reflected from the rear portion of the reflector converge on a point situated close to the glass.
Finally, mention should be made of foglights which are generally obtained using similar constructions to those used for American cut-off dipped beams, and which therefore generally suffer from the same drawbacks.
Thus, regardless of the type of cut-off beam that may be desired, and regardless of the practical techniques used to obtain it, prior art reflectors all tend to provide excessive light concentration in the center of the closure glass.
The prior art also includes the Assignees' French patent No. 2 528 537 which describes a European standard dipped beam headlight having a dipped filament provided with a masking cup, a reflector of the parabolic type with its focus in the vicinity of the filament, and a closure glass. In this prior patent, the rear portion of the reflector is modified in particular to avoid excessive light concentration in the center of the glass. More precisely, the rear portion is a paraboloid like the remainder of the reflector, but at least one of its parameters is different.
Although such a solution does indeed reduce heating in the center of the glass, it nevertheless suffers from the drawback whereby the surface of the reflector than has zero order or first order discontinuities which complicate the manufacture thereof and which give rise to optical defects in the resulting beam.
Another drawback of the headlight described in this prior patent is that it is exclusively limited to filaments associated with a masking cup. Using a reflector with a modified parabolic rear portion in conjunction with a filament that does not have a masking cup would have disastrous consequences on the formation of the cut-off.
The present invention thus seeks to mitigate these drawbacks of the prior art and to provide a headlight which is suitable for emitting a beam including a cut-off from a filament which may optionally be provided with a masking cup, and which does not suffer from any problem of excessive heating in the center of the glass, without degrading the light yield.
Another aim of the invention is to achieve this result with a reflector which does not have any discontinuities which are observable in zero order or in first order.
A further aim of the invention is to simultaneously provide a cut-off light beam which, in the absence of the closure glass, already has substantial sideways spread, thereby minimizing the sideways spreading that needs to be provided by the closure glass. As explained below, this feature makes it possible to use closure glasses at a very steep slope.
Finally, a subsidiary aim of the present invention is to provide a headlight in which determined zones of the glass have light rays passing therethrough corresponding to images of the filament whose sizes lie within respective given ranges, thereby enabling the glass to influence some of the properties of the beam independently from its other properties.